Ink jet printing is a non-impact method that in response to a digital signal produces droplets of ink that are deposited on a substrate such as paper or transparent film. Ink jet printers, especially thermal or bubble jet drop-on-demand printers have found broad application as output for personal computers in the office and the home.
A major concern with all ink jet printing devices is pluggage of nozzles during operation and between operations. A critical measured property for an ink jet ink is the "decap time" which is the length of time over which an ink remains fluid in a pen opening when exposed to air and therefore capable of firing a drop of ink at its intended target. "Decap" means the pen is uncovered or "decapped". The major cause of decap failure is due to evaporation of the aqueous carrier medium, i.e., either cosolvent or water. Initial evaporation generally causes an increase in viscosity which affects the ability of the nozzle to fire a drop since ink jet pens are designed to operate within specific viscosity ranges. Initial failure may be a displacement from the intended target position caused by deflection of the drop or creation of an altered trajectory. Continued evaporation may result in a solid component being precipitated or crystallized in the nozzle which in turn could result in an undesirable satellite spray occurring or a single line of print diverging into a double line. Finally, the nozzle may form a solid skin and totally fail by no longer ejecting ink.
The situation becomes more complex with pigmented inks because a change in solvent composition due to evaporation can cause pigment flocculation or aggregation. This will also lead to nozzle pluggage. In a multinozzle printer, certain nozzles are assigned to infrequently used characters and these are the most likely to undergo pluggage failure.
A further complicating factor is the use of heating devices to accelerate ink drying on the paper. This also accelerates solvent evaporation in the nozzle and makes pluggage more likely.
When the printer is lying idle, it is desired that an ink that has been exposed to air for several hours inadvertently or otherwise should print satisfactorily. If such a nozzle fails to print it is desirable that the nozzle become operational after firing several times into a "spittoon". The ability to recover in this manner is measured by fifth and thirty-second drop decap time test.
Various means of increasing decap time have been developed. As already mentioned above, one involves non-printing ejection, i.e., firing ink into a "spittoon" after a certain time interval of a nozzle not printing. This tends to slow the printing rate. Another decap cure is the use of forced air or vacuum suction to clear a plugged nozzle. These devices are often inefficient and add considerable expense to the cost of the printer.
Pluggage can be retarded by use of high boiling cosolvents. Use of high boiling water soluble liquids such as triethylene glycol and glycerol do not produce the desired improvement in decap time and also retard drying on paper as shown in Tables 2 and 4, Controls 5 and 6, in the Examples.
An important requirement for inks where the colorant is a pigment is for the pigment dispersion to remain stable throughout the life of the ink jet cartridge. Dye-based ink jet inks suffer from deficiencies in water fastness, smear fastness and lightfastness. Therefore pigments are a preferred alternative to dyes provided the pigment dispersions can be made stable to flocculation and settling. Some cosolvents that are good pluggage inhibitors cause destabilization of pigment dispersions and therefore cannot be used as pluggage inhibitors in pigmented inks. An example of this is 1,2,7,8-octanetetrol as evidenced in Table 3, Control 7.
Accordingly, a need exists for cosolvents which function as pluggage inhibitors without destabilizing pigment-based inks and excessively increasing drying time on paper. The pluggage inhibitors claimed in this invention have the ability to retard or inhibit print failure due to pluggage formation, demonstrate compatibility with aqueous pigment dispersions, and have acceptable drying rates. These pluggage inhibitors however, are also useful in dye-based inks.